The Boomerang Nebula (ESO 172-7) is a protoplanetary nebula located approximately 1,213 light-years away in the constellation Centaurus. Also known as the Centaurus Bipolar Nebula, it is currently the coldest known place in the universe. It has a temperature of only 1 K (-272.15°C or -458°F).
The Boomerang Nebula formed when a star in the late stage of its life shed its material, forming clouds of dust and gas. The hot stellar core illuminates the expanding clouds, revealing the complex surrounding nebulosity. The Boomerang is currently evolving towards the planetary nebula phase.
The preplanetary nebula stage is a very brief one in the evolution of stars. It only lasts about a thousand years. When the central star becomes hot enough to produce enough ultraviolet radiation to ionize the surrounding clouds, the Boomerang will become a planetary nebula.
The Boomerang Nebula, called the “coldest place in the universe,” reveals its true shape to the Atacama Large Millimeter/submillimeter Array (ALMA) telescope. The background blue structure, as seen in visible light by NASA’s Hubble Space Telescope, shows a classic double-lobe shape with a very narrow central region. ALMA’s resolution and ability to see the cold gas molecules reveals the nebula’s more elongated shape, as seen in red. Image: ALMA (ESO/NAOJ/NRAO)/R. Sahai (CC BY 4.0)
The nebula’s structure consists of a large, round molecular gas cloud which is rapidly expanding and a considerably denser cloud of dust surrounding the central star. The smaller, doughnut-shaped dust cloud blocks the light radiating from the star’s equator, making the nebula appear double-lobed.
Parts of the Boomerang Nebula’s central region are obscured by dust. The tiny grains of dust that mask portions of the nebula’s centre cause the Boomerang to appear hourglass-shaped when seen in visible light from Earth. The visible light that escapes from the central star illuminates the two opposing lobes that form the hourglass.
The complex structure of the Boomerang Nebula also includes a hollow cylindrical nebula around the star in its final stages. The hollow portion is suspected to have been formed by jets of hydrogen or helium gas emanating from the star’s poles. These jets are eroding the surrounding gas and carving out a tunnel in it.
The Hubble Space Telescope has “caught” the Boomerang Nebula in these images taken with the Advanced Camera for Surveys. This reflecting cloud of dust and gas has two nearly symmetric lobes (or cones) of matter that are being ejected from a central star. Over the last 1,500 years, nearly one and a half times the mass of our Sun has been lost by the central star of the Boomerang Nebula in an ejection process known as a bipolar outflow. The nebula’s name is derived from its symmetric structure as seen from ground-based telescopes. Hubble’s sharp view is able to resolve patterns and ripples in the nebula very close to the central star that are not visible from the ground. Image credit: NASA, ESA and The Hubble Heritage Team (STScI/AURA) (PD)
The central star is believed to be a star system. As it continues to expel material from its surface and the outflowing gas continues to move outwards, the nebula continues to form. The outflowing gas expands at a rate of 164 km/s. The expansion of the gas results in the nebula’s exceptionally low temperature.
The Boomerang Nebula was discovered to be the coldest known natural place in the universe in 1995. Astronomers Raghvendra Sahai and Lars-Åke Nyman used the 15-metre Swedish-ESO Submillimetre Telescope at the La Silla Observatory in Chile and found a temperature of -272° C, or only 1 degree kelvin. This is even cooler than the cosmic microwave background (CMB), the background glow (thermal radiation) left over from the Big Bang. The CMB has a temperature of 2.72548 ± 0.00057 K (-270° C).
Observations with the Atacama Large Millimeter/submillimeter Array (ALMA) in the Atacama Desert of Chile in 2013 revealed that the nebula’s double lobe was surrounded by a large volume of cold gas that could only be detected in sub-millimetre radio wavelengths. The radio interferometer showed that the outer regions of the nebula appear to be slowly warming but are still colder than the cosmic microwave background.
Boomerang Nebula, a protoplanetary nebula, as seen by the Atacama Large Millimeter/submillimeter Array (ALMA). The background purple structure, as seen in visible light with the NASA/ESA Hubble Space Telescope, shows a classic double-lobe shape with a very narrow central region. ALMA’s ability to see the cold molecular gas reveals the nebula’s more elongated shape, in orange. ALMA looked at the nebula’s central dusty disc and the outflows further out, which span a distance of almost four light-years across the sky. These outflows are even colder than the cosmic microwave background, reaching temperatures below –270 °C. The outflows are also expanding at a speed of 590,000 kilometres per hour. Image credit: ALMA (ESO/NAOJ/NRAO)/R. Sahai (CC BY 4.0)
A study published in 2017 confirmed that the Boomerang Nebula is the coldest known object in the universe. Based on ALMA observations, a team led by R. Sahai, W. H. T. Vlemmings and L-Å. Nyman studied the distant regions of the cold outflow, stretching for over 120,000 astronomical units (AU), or at least 80 arcseconds. The ultra-cold outflow has a mass of around 19 solar masses. The researchers reported that the nebula’s dense central waist was around 1,740 by 275 AU across and contained large, cold grains of dust.
The inferred mass of the outflow is too large if the nebula has evolved from a red giant (RGB or AGB star). The stars that produce planetary nebulae typically have a main-sequence mass of up to 8 solar masses. Astronomers believe that the mass loss rate is not uniform.
The astronomers derived expansion ages of up to 1,925 years for the central waist and up to 1,050 years for the fast bipolar outflow, with a lower limit of 3,480 years for the ultra-cold outflow. The difference in the expansion ages supports the belief that the primary star interacts directly with a binary companion. The interaction resulted in the formation of a common envelope when the central star was on the red giant branch (RGB) or early asymptotic giant branch (AGB). Models suggest that the companion merged into the primary star’s core, which resulted in the ejection of the primary star’s outer envelope.
The Boomerang is notable for its large mass loss rate (around 0.001 M☉ yr−1) – about 10 times larger than the highest rates seen in other AGB and post-AGB stars – and an anomalously low luminosity (only 300 L☉). The central star of the Boomerang Nebula has lost almost 1.5 solar masses of material in the last 1,500 years.
Combined with its unprecedented temperature, these properties make the Boomerang one of the most popular targets for studies. Astronomers study the nebula to gain insights into the mass loss of stars that have evolved away from the main sequence and the mechanisms responsible for producing bipolar morphologies in planetary nebulae.
The Boomerang Nebula by the Hubble Space Telescope, image credit: NASA/Hubble Team/John Biretta/Kevin M. Gill (CC BY 2.0)
Facts
The nebula was named the Boomerang by Keith Taylor and Mike Scarrott in 1980. The two astronomers observed it with the 3.9-metre Anglo Australian Telescope at the Siding Spring Observatory near Coonabarabran in New South Wales, Australia. They were only able to see the brightest sections and the slightly curved shape of the nebula’s asymmetrical lobes resembled the shape of a boomerang.
The Boomerang Nebula is sometimes called the Bow Tie Nebula. However, it is not to be mistaken for the Bow Tie Nebula (NGC 40, Caldwell 2), a planetary nebula in the northern constellation Cepheus.
In 1998, images taken by the Hubble Space Telescope revealed that the boomerang or bow tie shape was an illusion. They revealed more details of the nebula’s structure, showing that the cloud of gas surrounding the central star was in fact spherical. The Hubble image captured the nebula’s hourglass shape.
This NASA/ESA Hubble Space Telescope image shows a young planetary nebula known (rather curiously) as the Boomerang Nebula. Planetary nebulae form around a bright, central star when it expels gas in the last stages of its life. The Hubble telescope took this image with the Wide Field Planetary Camera 2 instrument in 1998. It shows faint arcs and ghostly filaments embedded within the diffuse gas of the nebula’s smooth ‘bow tie’ lobes. The diffuse bow-tie shape of this nebula makes it quite different from other observed planetary nebulae, which normally have lobes that look more like ‘bubbles’ blown in the gas. However, the Boomerang Nebula is so young that it may not have had time to develop these structures. Why planetary nebulae have so many different shapes is still a mystery. The general bow-tie shape of the Boomerang appears to have been created by a very fierce 500 000 kilometre-per-hour wind blowing ultracold gas away from the central star. The star has been losing as much as one-thousandth of a solar mass of material per year for 1500 years. This is 10-100 times more than in other similar objects. The rapid expansion of the nebula has enabled it to become the coldest known region in the Universe. Image credit: ESA/NASA (PD)
Location
The Boomerang Nebula lies in the constellation Centaurus, near the border with Crux. It appears in the region of the Southern Cross, one of the best-known asterisms in the far southern sky. The nebula can be found roughly halfway between Mimosa (Beta Crucis) in Crux and Muhlifain (Gamma Centauri) in Centaurus. Muhlifain appears along the imaginary line extended through the axis of the Southern Cross.
The location of the Boomerang Nebula, image: Stellarium
At declination −54° 31′, the Boomerang Nebula is invisible to observers north of the latitude 34° N. It never rises high above the horizon for observers in the northern tropical latitudes and is best seen from the southern hemisphere.
The best time of the year to observe the Boomerang Nebula and other deep sky objects in Centaurus is during the month of May, when the constellation appears higher above the horizon in the early evening.
Boomerang Nebula
| Constellation | Centaurus |
| Object type | Protoplanetary nebula |
| Right ascension | 12h 44m 46.0915042296s |
| Declination | −54° 31′ 13.329460776″ |
| Apparent size | 1′.445 × 0′.724 |
| Distance | 1,213 ± 60 light-years (372 ± 18 parsecs) |
| Radius | 1 light-year |
| Names and designations | Boomerang Nebula, Centaurus Bipolar Nebula, Bow Tie Nebula, PGC 3074547, LEDA 3074547, ESO 172-7, HBC 592, IRAS 12419-5414, 2MASS J12444609-5431133, GN 12.41.9, TIC 419500551, UCAC4 178-100510, AKARI-IRC-V1 J1244461-543113, AKARI-FIS-V1 J1244457-543115, WISE J124446.09-543113.2, WISEA J124446.09-543113.2, WKK98 1328, Gaia DR2 6073662099660289536, Gaia DR3 6073662099660289536 |